1. Field of the Invention
This invention relates generally to series/parallel LED drive systems, and more particularly to LED drive systems designed to accommodate multiple dimming inputs.
2. Description of the Related Art
LEDs (light emitting diodes), often connected in series to form LED ‘strings’, are being used to provide light in ever more challenging systems. The drive systems used to control the current conducted by the LEDs often require multiple means to dim (or brighten) the LEDs. This is accomplished by decreasing or increasing the average LED current. LEDs are dimmed for various reasons, including, for example, aesthetics, fault protection, and power savings. Newer LED systems require more and more means to dim the LEDs.
There are several problems that may arise for LED systems having multiple dimming inputs. For one, more than 2 dimming inputs may create a control problem for managing the LED current. Some systems control an LED's brightness by pulse-width modulating (PWM) its current; problems with this approach may occur due to the noise generated by the modulation, and/or due to difficulties combining the PWM signal with other dimming control signals.
One approach is described in National Semiconductor's LM3464 datasheet. Here, 2 dimming signals are combined to output a PWM'd LED current. However, this technique may produce erratic signals (noise) and/or beat frequencies that may create visible lighting patterns. To mitigate these potential problems, it is recommended that one dimming signal be high frequency and one be very low frequency. But PWM dimming at low frequencies (<20 kHz) can create noticeable audible noise due to the high pulsed powers involved. The problem can become exponentially worse if another PWM dimming signal is added.
Many LED control ICs provide one PWM dimming input and one analog dimming input. One example is shown in Texas Instrument's TLC5960 datasheet. Here, the LED current is controlled by an operational amplifier that receives an analog voltage (VADJ) at one input and a reference voltage at its other input to set a DC current, while another signal pulses the output current at a particular duty cycle. However, this scheme is unable to handle multiple inputs, as the reference voltage would become unreasonably small.